An electrochromic material changes its optical properties in response to an electrically driven change in its state of oxidation or reduction. In order to make use of the optical modulation in practical applications, the electrochromic material can be incorporated into a multilayer coating. The multilayer structure provides a means of supplying electronic current over the optical switching area of the coating and a counter electrode layer which undergoes reversible reduction and oxidation reactions. The counter electrode provides a means for charge-balancing the reduction-oxidation reactions in the electrochromic film and is necessary to achieve a large number of reversible optical switching cycles. An ion conducting layer with a high electronic resistivity separates the electrochromic film from the counter electrode layer. During the optical switching process, counter ions are transported between the electrochromic and counter electrode layers to preserve charge neutrality within the electrochromic coating. The counter ions are usually H.sup.+ or Li.sup.+. Structures of electrochromic devices incorporating these basic elements have been described in prior art patent and scientific literature. Examples include: thin film coatings using H.sup.+ ion transport between WO.sub.3 and IrO.sub.2 (Takahashi et al., U.S. Pat. No. 4,355,414, September 1992); and, thin film coatings using Li.sup.+ ion transport between WO.sub.3 and Li.sub.y CrO.sub.2+x (Cogan and Rauh, U.S. Pat. No. 5,080,471, June 1992).
In many applications, electrochromic coatings will encounter elevated temperatures, high levels of solar irradiance, and a wide range of humidities. Chemical, photothermal, and photochemical degradation may accompany these environmental stresses. Of particular importance in determining the severity of degradation is the role played by water (H.sub.2 O) in the layers in the electrochromic coating. Both hydration and dehydration of the electrochromic coatings during service may result in degradation and decline in useful switching performance. For example, in electrochromic coatings using Li.sup.+ ions, the layers are often hygroscopic and absorb H.sub.2 O from the ambient air. The absorbed H.sub.2 O reacts with the lithiated electrochromic layers causing a decrease in optical switching range and, in coatings for visible transmittance or reflectance modulation, also the appearance of haze. In electrochromic coatings employing the H.sup.+ ion, loss of H.sub.2 O by dehydration in dry atmospheres may result in a severe reduction in optical switching speed. The reduction in speed is due to a decrease in H.sup.+ ion conductivity of the electrochromic and ion conducting layers which must be partially hydrated to achieve the desired level of ionic conductivity.
If a thin film electrochromic coating is to have a useful lifetime in any practical application, a means of preventing exchange of H.sub.2 O between the electrochromic coating and the ambient environment is necessary. Although there is an extensive body of literature on thin-film coatings for protecting integrated circuits and optical elements such as filters and lenses, there is very little prior art concerning the protection of electrochromic coatings exposed to the environment. Effectiveness as a barrier to H.sub.2 O transport is a key property of a protective overlayer for electrochromic coatings. However, prior art coatings used to protect integrated circuits and optics have not demonstrated appropriate optical properties in combination with the required hermeticity to H.sub.2 O transport.
Optically, the protective overlayer must have a high transmittance in the wavelength range for which the electrochromic coating is being used to modulate radiation. For coatings used in visible modulation, a high transmittance in the 400-700 nm wavelength range is required while coatings for solar modulation must be transparent from 350-2500 nm. Transparency at wavelengths shorter than 350 nm is usually not required since it is often desirable to prevent transmission of the solar ultraviolet. Other desirable properties of the overlayer include a high electronic resistivity to prevent the overlayer acting as an electronic short between the transparent conductors or the electrochromic layer and counter electrode; a high hardness to provide protection against abrasion; and low intrinsic film stress to prevent delamination of the coating.
A variety of materials have been investigated as thin-film hermetic coatings. In integrated circuit (IC) fabrication, dielectric passivation, such as silicon dioxide or silicon nitride, deposited by low pressure chemical vapor deposition (LPCVD) or plasma enhanced chemical vapor deposition (PECVD), is typically used as a barrier layer to H.sub.2 O and ion migration. For most IC devices, an additional encapsulant such as an epoxy is used in conjunction with the dielectrics. As will be shown by example, conventional semiconductor passivation has either inadequate H.sub.2 O barrier properties or inappropriate optical properties for overlayers on electrochromic coatings. IC passivation is also deposited at elevated temperatures, &gt;350.degree. C., which cannot be tolerated by most electrochromic coatings, particularly those relying on H.sub.2 O Of hydration to maintain a high H.sup.+ ion conductivity. Other materials such as amorphous silicon carbide (Hiraki, et al U.S. Pat. No. 4,647,472, March 1987) have been described for semiconductor passivation. Amorphous silicon carbide has excellent properties as a H.sub.2 O barrier but, as will be shown by example, has poor transmittance in the visible region of the spectrum.
Prior art methods of protecting electrochromic devices using H.sup.+ ions have been described by Agrawal et al (U.S. Pat. No. 5,216,536, Jun. 1, 1993) and Shabrang (U.S. Pat. No. 5,136,419, Aug. 4, 1992). Agrawal et al reveal a moisture control layer comprised of a water reservoir material in conjunction with a moisture permeable barrier layer that prevents dehydration of the electrochromic coating. This approach to preventing dehydration has the disadvantage of limited optical transmittance due to the reservoir and barrier materials. The improvement described by Shabrang is to operate an electrochromic coating in an atmosphere containing an inert gas and vapor of high dielectric constant material (for example, water vapor) enclosed in a double-pane window in which the electrochromic coating is on an interior surface. This approach is deficient in that it is limited to a double-pane configuration and that, at low temperatures, the vapor will condense on the interior surface thereby compromising optical clarity and promoting chemical degradation.
The invention described, herein, is intended to overcome the deficiencies of prior art approaches to protecting electrochromic devices and to provide a means of protecting both H.sup.+ and Li.sup.+ counter ion electrochromic coatings against H.sub.2 O egress and ingress for periods of time appropriate for commercial applications that include, but are not limited to, electrochromic displays, sunglasses, automobile sunroofs, and large-area architectural glass for building windows.